The present invention relates to a communication system between at least one CS concentrator connected to at least one PHS radio base station (hereinafter referred to as CS) and located in a remote area and a subscriber terminal device in a PHS exchange.
In a conventional PHS system, connection is made by a 1.5M primary-group transmission line between a CS concentrator containing a plurality of CSs and having a line concentrating function and a subscriber terminal device in a PHS exchange. The 1.5M primary-group transmission line is, as disclosed in Bellcore TR-TSY-000303, a system for time division multiplexing of (2B+D) channels of an ISDN basic interface (hereinafter referred to as BRI) on time slots and for multiplexing a maintenance channel C in a channel D time slot.
Also, as disclosed in TTC JT-Q931-b, it is well known that there is a mode having an interface structure for a shared channel D. In this case, a certain BRI channel is used as a control channel of a channel B in another BRI. The mode has the interface structure of (16B+D) which uses eight BRIs.
Further, as a traffic control method of the conventional PHS exchange, the following procedure is known. In the procedure, in response to a calling/position registration message which is generated beyond a processing capability and defined by TTC JT-Q931-b, a disconnection message is returned from the exchange back to the CS concentrator.
The aforementioned 1.5M primary-group transmission line is not an economical loop operation method. This is because in the system using the conventional 1.5M primary-group transmission line, the call control of sixteen channels B in eight BRIs is performed by a channel D of a single BRI and seven channels D are therefore unused.
Also, in the conventional traffic control method, in response to the calling/position registration message, the disconnection message is returned from the exchange back to the CS concentrator. In this case, when the number of CS concentrators is increased, there is a possibility that not only a new calling/position registration but also a call in the midst of a call setting procedure are disconnected by time out of a timer (T310) of layer 3 because of a delay between the CS concentrators and the exchange and a delay inside the exchange. Specifically, in the conventional traffic control method, when the number of CS concentrators is increased and the call information beyond the processing capability is received, then the information is placed in a queue for processing. Therefore, the more the quantity of call information is, the longer the waiting time in the queue is.